1. Field of the Invention
This invention relates to an image processing apparatus, method and controller for controlling density.
2. Description of the Related Art
FIG. 18 is a block diagram illustrating the construction of a printer apparatus according to the prior art. The printer proper, indicated at 202, is connected to a host computer (hereinafter referred to as a "host"), which is an information source of image information to be printed.
The printer 202 comprises a controller 206 and an engine 207, which receives print information outputted by the controller 206 and prints the data on recording paper. The controller 206 comprises an interface I/F 203 for connection to the host 201, a frame buffer 204 for retaining data, which has been transferred from the host 201, as image data to be printed, and a reproduction unit (hereinafter referred to as an "RF unit") 205, which subjects the output of the frame buffer 204 to masking and UCR processing to convert the output to a signal suited to the engine 207.
The printer 202 constructed as described above will now be described. Multivalued image data that has entered from the host 201 is applied to and retained in the frame buffer 204 via the interface 203. At the time of printing, the image data is read out of the frame buffer 204 in sync with the recording speed of the engine 207. The image data read out is converted by the RF unit 205 in conformity with the characteristics of the engine 207. By way of example, if the image input data has entered in the form of an RGB signal, the data is converted to a signal capable of being handled by the engine 207, namely YMCK four-color image data in a case where full-color printing is performed using the colors Y, M, C, K. At this time masking processing or UCR processing, which has been optimized for the process characteristics of the engine 207, namely the toner characteristics and development bias--density characteristic, is executed. The engine 207 prints out the image data that has undergone the aforesaid processing.
In this multivalued image recording apparatus, it is required that the relationship between the entering density-level signal and the density actually printed be linear, and it is necessary that he density printed be constant with respect to the same density-level signal regardless of temperature and humidity. In an electrophotographic printer, however, fluctuations in the toner characteristic and development bias--density characteristic make it difficult to maintain density linearity and consistency. In general, automatic density control for each of the colors Y. M, C, K is carried out in the engine.
The following problems are encountered in the prior art described above:
In the arrangement described above, the color conversion characteristics of masking and UCR in the RF unit 205 of the controller 206 must have one-to-one correspondence with the process characteristics of the engine 207. Consequently, when the controller 206 is developed, for example, the start of development must wait for settlement of the engine process characteristics, as a result of which the development period is prolonged. Furthermore, image processing functions inclusive of a color converting function cannot be added onto the controller, and therefore the manufactured product cannot be provided with additional value.
More specifically, in the engine 207 which has a construction independent of that of the controller 206, the density of the print image is fixed at a density characteristic which seems to be ideal. Accordingly, image processing carried out in the controller 206 before data is outputted to the engine 207 is not faithfully reflected in the final output image, and tones or colors that satisfy various user needs satisfactorily cannot be expressed.